TheSamba.com :: View topic - Any ideas on diagnosing burnt valve on the road?

Tue Aug 30, 2016 3:25 pm

I am traveling through southern europe, temperatures were quite hot for the engine and i've developed an exhaust sound, but i don't see any leaks. Engine ran a little rough so I adjusted valves and found both for #4 cylinder a little tight. Engine ran fine after that, sound still present, did about 700 km after that. Now in Madrid starting to suspect a burnt exhaust valve becase i had a pretty loud bacfire on starting.

I don't have a compression or leak tester with me and Spaniards aren't that well acquainted with these engines. I still have a long way to go home. I have a spare head with me, but it's not something I'm looking forward on replacing in the street if i don't really have to. Would anyone have an idea how i could confirm or dismiss my theory of a burnt valve using basic tools?

(type1 1600 all stock late bay)

Tue Aug 30, 2016 3:29 pm

You can adjust the valves then do a cylinder balance test to see if the cylinder is weak. If it is not then I would suspect a stretching valve or sinking seat. Both are bad news.

Tue Aug 30, 2016 3:46 pm

That's a good idea to start with, although I don't feel any power loss or fuel economy.

I will re-check the lashes on #4 in the morrning. If the seats are sinking it's probably best to replace the head before it goes bang. I've seen a type4 engine go through a seat pretty quickly before. Not sure how prone type1s are.

Tue Aug 30, 2016 4:40 pm

A burnt valve doesn't really make an external "sound", if it still has compression on all 4 and slows equally when you pull each plug wire from the cap at idle the valve is doing it's job. A sinking seat or stretching valve can be detected by removing the rockers and putting a straightedge across the tips.

When it's idling and you block the tailpipe with a wet rag can you hear any exhaust leaks?, from your description that's where I'd begin.

Tue Aug 30, 2016 5:12 pm

FWIW - you indicated a little tight on one valve. I have no way of knowing what "a little tight" really means other than you noticed a change. If it is .0005" tight then that is not something that is overly concerned about. If it is consistently .0005" tight then it is moving. On the other hand if it was .002" to .003" tight then that is material. You have to be the judge of that. A seat moving around is going to sound a little like a loose lifter. If you adjust them again and there is no change you can do the cylinder balance test and if the noise is still there but the cylinders are even then it will almost for sure be an exhaust leak. Listening thru a piece of hose waved around the exhaust will for sure pick up any leaks. If you can't stall the engine with a wet rag balled up on the exhaust that will also show a leak. A tight exhaust will stall the engine at idle.

Tue Aug 30, 2016 5:34 pm

I have found exhaust leaks by accident when winterizing engines- if you have some one hold the idle at 2000 rpm and pour oil or ATF into the carb slowly till it smokes out the pipe, you will see every exhaust leak in the system- warn your neighbors so they don't call FIRE.

Wed Aug 31, 2016 12:19 am

It is not unusual for the valves on one cylinder to be .025mm tight when checked and then once reset be loose the same amount the next time they are checked.

0.050mm and I would begin to worry a bit, and any more than that and I would begin to watch the valves very carefully.

Wed Aug 31, 2016 2:26 am

Ok, it's morning in Madrid, I found the exhaust leak between the heat exchanger and muffler, so that was the sound, did a bit of a ghetto repair on that, but I still seem to have a sinking valve. In 700km (500miles?) since I last adjusted the #4 cylinder valves, the intake has dropped from 0.15mm to 0.05mm and the exhaust valve to 0.10mm. I took the rockers off and #4 intake was about 1mm higher (by eye) than the rest.

Put them all back to 0.15 again.

The dillema now is, do i keep going, checking on them every once in a while, or tear the engine out in the street. I have about 2700km (1700 miles) to get home, crossings two very mountainous areas. Engine runs fine otherwise.

Wed Aug 31, 2016 5:31 am

having a cylinder head for a spare- after a valve drops is not going to help you.

If you change it out now- all is good and go on your way- after the valve drops and your blocking a 2 lane road with a hole in the piston and crap contaminating the engine block -- your pretty much done with your vacation --$$$$

Wed Aug 31, 2016 8:27 am

Reason hurts. You are quite right.

Anyone have experience with pulling the head with the engine installed? I've done this on a type4 bay, mine's type1, the late kind with large tins around.

Wed Aug 31, 2016 8:55 am

You need to remove the rockers and put a straight edge across the valve tips to measure. 1mm by eye is hard to see.

On a Type I engine it would be more likely that the valve seat is sinking into the head.
Check it about every 500 miles and see if it continues to get tight.

It may be worn out threads on the adjusters?

Good luck
Tcash

Wed Aug 31, 2016 9:04 am

I would replace the head with what you have revealed this morning. Type 1's are more prone to dropping valves from stretching than sinking seats. On a type 1 in a late bay you need a lift to pull the engine. There should be a VW club in Madrid where someone can help you and offer a lift. You can be on your way in hours if you pull the engine. Trying to pull the head in the car will be very hard with an upright shroud unless it fits straight up thru the hatch. In fact you may even be able to have the head inspected and rebuilt with all new valves there in a couple hours if someone will make time for you in their shop. Then you will still have a spare.

Wed Aug 31, 2016 9:27 am

I have a jack and two supports, and all the tools to do it except a torque wrench, but i don't think it's a big deal to tighten a typ1 head by feel.
The spare head isn't new either.

Thanks for all your advice. We sleep here another night and either strip it in the morning or drive and see how it holds up. There's an aircooled guy in Zaragoza who helped me out before. If it keeps sinking, i can stop by there.

Wed Aug 31, 2016 9:38 am

Sven F wrote: I have a jack and two supports, and all the tools to do it except a torque wrench, but i don't think it's a big deal to tighten a typ1 head by feel.
The spare head isn't new either.

Thanks for all your advice. We sleep here another night and either strip it in the morning or drive and see how it holds up. There's an aircooled guy in Zaragoza who helped me out before. If it keeps sinking, i can stop by there.

Drive gently.

Thu Sep 01, 2016 10:15 am

Drove to Zaragoza today really slowly. Dropped my cruising speed from 90 to 75 kph. Kept the oil temp below 120°C Stopped to cool off, checked valves on #4 All good! I'll try driving at night when ambient temperatues drop until I get up to Switzerland. I think the hi heat combined with too much ethanol in spanish fuel isn't helping.